The present invention relates to a new and improved method of, and apparatus for, improving the flushing or backwashing of fixed beds composed of granular materials, these fixed beds being typically employed during sewage treatment or waste water purification or water treatment. Examples of the aforementioned fixed beds or granular package are, for instance, sand filters, activated charcoal-adsorbers and ion exchangers.
In the context of the disclosure of this invention, the term "backwashing" or "backflushing" is used in its broader sense to embrace flow of the backwashing liquid not only counter-current to the treated liquid but also substantially in the same direction of the flow thereof.
The backwashing of fixed beds, for example rapid sand filters utilized for instance during purification of drinking water and treatment of waste water and through which filter the treated liquid, during operation, can flow upwards, downwards, or radially, generally is accomplished with air, air-water and/or purely by means of water. A great number of filter constructions are provided with mechanical devices for loosening or disintegrating the sand of the filter bed.
During the treatment of surface waters to obtain potable or drinking water and in the more extensive purification of waste water or sewage treatment there is oftentimes formed a type of algae-suspension-slime or activated sludge-suspension-slime. There is especially noted the effect of so-called mud ball formations. The sand granules stick together to form hard conglomerates, the diameter of which can reach as much as several centimeters and these sand grain-conglomerates are usually not capable of being disintegrated by the normal flushing operation. Generally, these mud balls predominantly consist of organic substances and microorganisms and additionally contain sand, further inorganic components, such as for example iron and calcium as well as nitrogen and oxygen.
A further effect is observed during waste water filtration as concerns the change in the structure of the sand bed. The sand grains or granules can be employed as the condensation core for coating biological growth and, for instance, can grow from an original diameter of 1 millimeter to a diameter up to 5 millimeters. This process is especially encountered in the case of sand filters which are employed at sewage disposal plants working with trickling filters.
Such mud ball-formations and/or the pronounced grain encasing-biological growth produce excessive head losses in the filter, resulting in shorter filter operating times. Moreover, an intense formation of mud balls impairs the quality of the filtration operation and is associated with the danger of breakthrough of the suspension, i.e. a rapid depletion of the filter capacity in holding back suspensions due to the presence of the mud balls which have not been washed out during filter backwashing.
A prior art technique of combating this effect which disturbs sand filtration contemplates employing extremely high flushing or backwash velocities (90-160 m/h) during backwashing with water and/or with a combination of water and air. The disadvantage of both of these backwashing techniques resides in the fact that an increase of the velocity of the backwashing liquid is not possible for technological reasons in the case of installations which are already in existence. With regard to newly constructed installations, water backwashing velocities V.sub.R &gt;60 m/h are associated with technological process problems, requiring the construction of backwash water-storage basins which, in turn, lead to high additional investment- and operating costs. Experience has shown that backwashing with a water-air mixture leads to high losses in sand and there is not positively insured for the desired cleaning affect. Additionally, in existing plants the installation of devices which are needed for the backwashing operation and the conversion of the plant can only be carried out with great financial expenditure.
A further possibility of counteracting the formation of mud balls or the like and the grain coating-biological growth is to employ in the backwashing water oxidation agents, such as for instance peroxides, chlorine, as well as chlorine compounds (hypochlorite, chlorite, chlorate) in order to kill microorganisms in the filter and by an oxidative action obtaining destruction of the mud balls. Yet, when using chlorine there is the hazard of forming carcinogens and organic chlorine compounds which are difficult to eliminate biologically; both during the preparation of drinking water and during waste water purification this can lead to difficulties as concerns the handling of the backwashing water.
Additionally, due to the use of chlorine or chlorine-containing substances there is also destroyed the desirable microorganisms contained in the sand filter. The problem of "chemical contamination" of the backwashing water and/or the filtrate is generally not present when adding peroxides into the flow of the backwashing water, but throughflow backwashing where a peroxide (i.e. a weak oxidation agent) is added as the oxidizer to the backwashing water, has not proven to be completely satisfactory.